Low rank carbonaceous materials, such as brown coal, peat and lignite, are materials having water locked into a microporous carbonaceous structure. The water content is typically high—for example 60% or higher. This means that such raw materials have a low calorific value. Moreover, these materials have the undesirable mechanical properties of being soft, friable and of low density, meaning that they are difficult, messy and inconvenient to handle.
Prior processes for upgrading low rank carbonaceous materials (which for ease of discussion will be hereinafter collectively referred to as “brown coal”) in order to remove water and increase calorific value have included “briquetting” and solar drying.
Briquetting typically involves heating the raw brown coal to remove excess water, then pressing the cooled brown coal into briquettes using a press or roll briquetting machine. However, briquetting is energy intensive due to the need for thermal energy to heat the raw brown coal.
The solar drying process involves milling of the brown coal with addition of water, then solar drying of the milled slurry in shallow ponds. This process is lengthy—particularly the solar drying step which may take up to several months—and energy intensive.
Another proposal mechanically releases water from brown coal by physically breaking up the brown coal. However, this process is inconvenient and time consuming and still requires lengthy air drying of the final product.
WO 01/54819 describes an upgrading process which comprises subjecting brown coal to shearing stresses which cause attritioning of the microporous structure of the brown coal and release of water contained in the micropores.
The shearing-attritioning process is conducted at a nip defined between two or more converging surfaces, wherein at least one of the surfaces is rollable towards the nip. The two or more converging surfaces may comprise part of a pelletising machine, such as a rotating roll type pelletising machine. The shearing-attritioning is continued until the brown coal forms a plastic mass that can be simultaneously formed into pellets, then subsequently dried. The pellet formation may be by way of forcing (“extruding”) the mass through apertures in the wall of the pelleting machine. The moisture content of the formed pellets may be around 50-60%, depending on the provenance of the brown coal. Run of mine Loy Yang lignite, from Victoria, Australia typically contains around 65% moisture, which reduces to around 52% moisture after pellet formation.
All of the above upgrading processes, and particularly those involving the use of thermal energy applied through direct-drying applications, can suffer from the problem of dust generation during drying of the product, thereby requiring use of dust control steps, such as wet scrubbing or use of dust removal means including bag-house applications, which are inconvenient and expensive and can even be dangerous.
In the case of WO 01/54819, in order to accelerate drying of the upgraded brown coal pellets, hot air may be blown through the pellets. However, this can cause significant generation of dust and associated environmental pollution. Moreover, due to the pyrophoric nature of brown coal, hot air drying may also pose a significant risk of spontaneous combustion of the upgraded brown coal under some circumstances.
Another disadvantage of hot air drying is that evaporated moisture is lost. Given the current imperative to conserve water in industrial processes, it would be desirable to capture the evaporated moisture for other purposes.
The above discussion of the background to the disclosure is included to provide a context for the present disclosure. It is to be understood that such discussion does not constitute an admission that any of the material referred to was published, known or part of the common general knowledge in the art, in Australia or any other country.
It would accordingly be desirable to provide a process and an apparatus for drying material prone to generating dust, such as low rank carbonaceous material (which will hereon be collectively referred to as “brown coal” for ease of discussion), which overcomes, wholly or partly, one or more disadvantages of the prior art.